A boost converter is a power converter where a smaller input DC voltage is increased to a desired level. A prior art typical boost converter 10 is shown in FIG. 1. Boost converter 10 has input terminal 2, 4 for enabling an input voltage Vin to be coupled to converter 10, and output terminals 6, 8 where the output DC voltage is provided. The boost converter 10 includes an inductor 18, to which the input voltage Vin is coupled, that is in series with a boost diode 16 connected to an output capacitor 12 across which the load (not shown) is connected at terminals 6,8. A transistor switch 14 is connected to a node 15 between the inductor 18 and boost diode 16 and a ground return line 20 to provide regulation of the output voltage. The switch 14 is typically a MOSFET having a control input, a drain and a source terminal. A control circuit 42 (details not shown) is coupled to the control input for providing a control signal for controlling the timing of the on and off transition of the switch 14. The control circuit 42 typically has a pulse width modulation circuit (PWM). In operation, when the switch 14 is on, the inductor current increases, storing energy in its magnetic field. When the transistor switch 14 is off, energy is transferred via the diode 16 to the output capacitor 12 and the load. Drawbacks of such conventional boost converter circuits include the creation of switch voltage and current stresses resulting in low efficiency power conversion.
In a conventional boost converter, the voltage rating required for the MOSFET 14 is determined by the voltage appearing across the MOSFET when the control circuit 42 turns it off since the inductor 18 rises up to the output voltage plus any overshoot. A conventional boost converter with an output voltage of 250 VDC requires a MOSFET having a voltage rating (drain to source) of at least 400 volts. Conventional boost converter using in high step-up ratio applications, boosting 30V dc to 250V dc for example, therefore, require a high current and high voltage rated MOSFET. A drawback of the use of high current and high voltage rated MOSFETs is increased size and cost.
The voltage drop across a MOSFET 14 between the drain and source terminal is a function of the resistance (Rdson) provided that the load current is constant. Conduction losses for a MOSFET are equal to I2R losses, therefore, the total resistance between the source and drain terminals during the on state, Rdson, should be as low as possible. Consequently, a drawback of converters requiring higher Rdson rated MOSFETS is higher conduction losses.
FIG. 2 shows a prior art boost converter 20. The boost converter 20 adds a snubber circuit 24 to the boost converter 10 of FIG. 1. The snubber circuit 24 is designed to absorb energy from the leakage inductance in the circuit and transfer this energy to the output. The snubber circuit 24 includes a capacitor 22 connected in series between inductor 18 and diode 36 and ground return line 20. Snubber circuit 24 also includes a series combination of another inductor 28 and another diode 26 connected between the junction of the capacitor 22 and the diode 36.
In operation, when the MOSFET 14 in boost converter 20 switches from ON to OFF, the voltage at node 25 is clamped by capacitor 22 and diode 36 to terminal 6 before diode 16 is turned ON. As a result, the voltage at node 25 will be slightly higher than the output voltage at terminal 6. This causes diode 16 to turn ON and clamp the voltage of node 25 to Vout. When MOSFET 14 is ON, the charge (from the leakage energy) stored at capacitor 22 will flow through inductor 28 and diode 26. At that moment, capacitor 22 and inductor 28 form a resonant network that reverses the polarity of capacitor 22. More specifically, before MOSFET 14 is turned ON, the end of capacitor 22 at node 25 is positive, but becomes negative after MOSFET 14 turns ON and the resonant action completed. Thus, before MOSFET 14 turns OFF, node 25 is negative relative to the other end of capacitor 22. As a result, the voltage spike at node 25 is clamped more effectively.
A drawback of converter 20 is that it is not suitable for high power or high boost ratio applications. More specifically, in high power applications or high boost ratio applications, the switching current at MOSFET 14 becomes high and the added capacitor 22 must be able to handle the high current demands, making it more difficult to find a suitable capacitor for a particular application. Moreover, in high boost ratio applications, a high current and high voltage rated MOSFET 14 is still required for converter 20 since the MOSFET 14 drain terminal is still clamped to the output of the boost converter. Although an improvement over converter 10 of FIG. 1, converter 20 has the similar drawback of having an unacceptably high voltage spike (>Vout) at the drain of MOSFET 14 for high current applications so as to require higher voltage rated MOSFETS with higher Rdson and corresponding higher conduction losses.
A need therefore exists for a boost converter topology for high power applications to enable the use of lower voltage and lower Rdson rated MOSFETs and to provide increased efficiency by reducing conduction losses.